This invention relates to methods for reducing vertical segregation in high intensity discharge (HID) lamps, and more particularly, to a method for color mixing of HID lamps operated at very high frequencies which employs duty cycle modulation for operation of a half bridge circuit topology of the resonant inverter.
High intensity discharge lamps are not typically operated on high frequency sinusoidal waveforms due to instabilities caused by acoustic resonances. In an attempt to overcome such instabilities, the HID lamps can be stabilized by operating the HID lamp at frequencies above its highest acoustic resonance. Operating the HID lamp above its highest acoustic resonance is known as very high frequency (VHF) operation. Nevertheless, the frequencies of the acoustic resonances depend on a variety of factors, one of which includes the dimensions of the lamp and thus varies from one HID lamp to another HID lamp.
Moreover, when HID lamps are operated in a vertical orientation, certain HID lamps display a phenomenon called vertical segregation whereby the color of the light emitted from the HID lamp is not uniform along its vertical axis. As a result, attempts have been made to overcome the disadvantage of vertical segregation. In one such attempt, the second longitudinal acoustic mode is excited to mix the chemistry in the lamp yielding a uniform color along the vertical axis of the HID lamp.
As can be readily seen, it is desirable to operate the HID lamp at VHF and also excite the second longitudinal acoustic mode to create color mixing or, in other words, to minimize, if not essentially eliminate, vertical segregation.
The present invention contemplates a method of color mixing which includes operating an HID lamp at VHF, and power modulating at a frequency of a second longitudinal acoustic mode of the HID lamp by modulating the duty cycle of a half bridge configured resonant inverter to excite the second longitudinal acoustic mode of the HID lamp to achieve color mixing along a vertical axis of the HID lamp. The method also contemplates the ability to maintain a constant switching frequency of the half bridge configured resonant inverter.